Lithium-ion batteries (LIBs) have been widely utilized in various applications especially consumer electronics such as laptop computers, mobile phones, and digital cameras, etc. Recently lithium-ion batteries have started to be used in automobiles due to their superior energy and power density.
LIBs generally include an anode, a cathode, a separator, and an electrolyte. The anode and cathode are separated from one another by a separator in order to prevent short circuit while maintaining ion conductivity.
A separator is conventionally a thin, porous, electrically insulating material having high ion permeability, good mechanical strength and long-term stability to the chemicals and solvents used in the system, for example electrolyte of the electrochemical cell.
Separator for use in high performance battery system must be safe since very large quantities of energy are stored in the fully charged state in the battery. These energies must not be released in an uncontrolled manner in the event of malfunctioning of the battery, such as overcharging or short-circuit, since this would lead to an explosion or ignition of the battery.
Generally, a typical organic separator consists of a composite film comprising a polyolefin-based substrate and an inorganic coating layer. A major disadvantage of these polyolefin-based separators is their low thermal stability limit. When the battery temperature exceeds 150° C. or lower, the organic separator rapidly shrinks. A cathode and an anode will directly contact to each other, to cause enlargement of short-circuited area. Therefore, such a separator is prone to cause battery short circuit and is generally not safe.
The electrodeposition of lithium metal on the anode is another cause of failure in lithium batteries. The growth of lithium dendrites can pierce the separator causing a short circuit. Therefore, thermal and mechanical stabilities of the separator are the most important factors for battery safety.
There are different types of materials which are suitable for membrane modification. The most common approach for improving the thermal and mechanical stabilities of a separator is to apply ceramic materials such as inorganic oxides on the surface of the separator to form a composite membrane. This inorganic composite membrane as the separator for Li-ion batteries has good thermal and mechanical stabilities.
However, there is a trade-off between the performance, such as porosity/transport properties, and mechanical robustness when designing safe battery separators. High separator performance is decisively dependent on the ion-conducting properties. The ion conductivity of the separator therefore has to be high. This will best be achieved with the separator having coarse pores and low thickness. But at the same time a separator having fine pores and a relatively high thickness offers desirable safety features.
Moreover, different safety features are required for anode and cathode in lithium-ion batteries. Prevention of the growth of dendrites from anode to cathode is required. Otherwise, it would cause internal shorting. In addition, a large amount of heat may be generated by decomposition of cathode having a high oxidation state. Although coated separator can improve safety of the battery, it is unlikely that specific needs of different electrodes can be satisfied with the same inorganic coating material. A coating having a small pore structure, which is adjacent to the anode, prevents the Li dendrites from contacting the cathode. Inorganic material having high porosity and low thermal conductivity is preferred for a coating in contact with the cathode.
U.S. Pat. No. 7,709,140 B2 has disclosed a separator with asymmetrical pore structures comprising a first porous layer and a second porous layer, wherein the first porous layer and second porous layer have different average pore sizes and are in contact with different electrodes. However, the separator is designed without compatibility consideration with respect to characteristics of different electrodes. Therefore, safety and performance of the battery are not optimized.
CN Patent No. 102388484 B has disclosed an asymmetrical separator comprising an anode side and a cathode side, wherein each of the anode side and cathode side comprises a filler, wherein the filler at the anode side comprises Al2O3 and the filler at the cathode side comprises polyvinylidene fluoride (PVDF). However, the separator is only designed to meet different requirements in terms of chemical or electrochemical stability needed on both electrodes.
It is one of the objectives of the present invention to overcome the disadvantages and problems mentioned above.